On Lee Cyclogenesis and Airflow in the Canadian Rocky Mountains and the East Asian Mountains

Abstract

A comparative study for the year 1958 of 259 cyclones in the lee of major mountain barriers in Fast Asia, and of 146 cyclones in the lee of the Canadian Rocky Mountains, shows complex multiple-frequency maxima of initial cyclogenesis in juxtaposition to these major mountain systems. The initial formation of most lee cyclones occurred under a different, cross-barrier flow in the mid-troposphere. The difluent characteristics of the upper-level flow appeared to be intensified by the mountains. On the other hand, appreciable development or intensification of the cyclones begins with the approach of an upper-level cold trough or low. In general, this intensification occurs in regions away from the principal cyclogenetic zones. However, formation and intensification do take place in the same region in some instances. About 41% of the lee cyclones sampled in East Asia were weak, local cyclones of orographic origin. In western Canada, only 25% of the cyclones belong into this category. Many cyclones that formed in the lee of the Tibetan Plateau did not acquire appreciable intensity as they tracked across China. Moreover, lee cyclones of moderate to high intensity were observed to be much less common in central and southern China than in North America.